You are watching: Distance light travels in a nanosecond

We see right into the pastWe all agree on the rate of lightSimultaneity is in the eye of the beholderTime slows downLengths shorten

Photo by Casey Horner ~ above UnsplashWe See right into The Past

Get a flashlight, was standing one foot far from a wall, direct the flashlight in ~ the wall surface and rotate it on. The irradiate beam will show up ~ above the wall, watch instantly. Actually, it wasn’t instant. It took approximately a nanosecond. A nanosecond is 10⁻⁹ seconds i.e. A billionth the a second. That’s too quick of an interval to be perceptible to a human brain and so that feels choose the beam from the flashlight struggle the wall surface instantly.

In a *billionth of a second* (nanosecond) light travels one foot. This is the moment it takes for the irradiate from objects approximately us in a room to reach us so that we watch them virtually instantaneously.

In a *millionth the a second* (microsecond) irradiate travels a thousand feet. The irradiate from a street light a few blocks far takes a microsecond or so to with us, which still seems instantaneous come us.

In a *thousandth that a second* (millisecond) irradiate travels 186 miles. That is approximately the distance between Seattle and Portland. Now of course our flashlight won’t be detectable in ~ such large distances (due to its short power). But the time the takes because that light come travel hundreds of miles becomes noticeable to united state in various other ways. For example, if you visit a website top top your device in Seattle and also the website is served from a data facility in Atlanta, the data transfer across the thousands of miles adds tens of milliseconds hold-up to the loading the the website (compared come if the website was served from a data facility in Seattle). In instance you’re wonder what data transfer has to do v light, digital data is transmitted as electromagnetic waves in optical fibers (light being electromagnetic tide within a particular band the frequencies that room visible come the human being eye).

In one *second* irradiate travels 186,000 miles or 300,000 kilometers. That’s approximately the distance to the moon. So once we look up at the moon in the night sky, we’re actually seeing the the way it was a second ago (not the the moon is changing every second, but still).

It takes about *8 minutes* because that the irradiate from the sunlight to reach us. If the sunlight were to explode right now (don’t worry, that’s not going to take place till an additional 5 exchange rate years) we will not know about it it rotates 8 minute after!

The following time girlfriend look up at the night sky, wonder at exactly how you’re looking method into the past. It takes 10s of thousands of years for the irradiate from clearly shows stars to reach us. Are several of those stars quiet around? and if we go beyond the naked eye and also consider stars we can detect v telescopes and also other instruments, well then we’re spring millions and also billions that years right into the past!

We all Agree ~ above The rate Of Light

The speed of irradiate is, stated in miscellaneous units, **1 foot per nanosecond** or **186 miles every millisecond** or **300,000 kilometers per second**. This is technically the speed in a vacuum (e.g. In space). Light travels a bit an ext slowly through media such as glass or water, yet that’s not pertinent for this discussion.

A an ext relevant problem is according to who perspective is the rate of light 300,000 kilometers per second? If ns on a relocating bus and there’s a round on the seat next to me, the sphere is stationary (has 0 speed) from my point of view. However as you stand on the street and see the bus drive by, you watch the ball as moving at the exact same speed as the bus. The speed of the ball is different depending on the observer. What around the headlight beam create from the bus? Does it traverse a foot every nanosecond from her perspective or mine? The answer, counterintuitively, is that the headlight beam has actually the exact same speed from your perspective and also my perspective! the is really different indigenous the case of a ball on a bus.

Photo through CHUTTERSNAP top top Unsplash

*The rate of light is the same from every observer’s perspective*, also when the observers are relocating rapidly loved one to each other. For completeness, I’ll mention the salient information that each observer have to be in one inertial frame (i.e. Non-accelerating) yet let that information not obfuscate the main suggest here. The constancy of the speed of irradiate from every observer’s view is a *fundamental legislation of Physics* (proposed through Einstein in the year 1905 in a theory known as distinct Relativity). And also boy go it lead to some mind-blowing impacts that us shall check out next.

In the Eye that the Beholder: Simultaneity

I (on a relocating bus) watch two things happening in ~ the very same time. Girlfriend (standing on the road) should likewise see those two things happening simultaneously, right? Wrong! This is a result of the speed of light being constant for every observers.

How have the right to we know this lose of simultaneity throughout different observers? Well, through some shenanigans on the bus. What happens as soon as inside the bus the driver switches on a light that is precisely half-way between the front of the bus and also the rear? Obviously, the light will take trip at the exact same speed (conventionally denoted by ‘c’) in the direction of the front and also the earlier and reach the two ends at the same time (in a few nanoseconds ~ the light to be switched on). If ‘Lₘₑ’ is the street from the bulb to each finish of the bus, then the light will certainly reach each end after a time Lₘₑ / c.

But what perform you view while standing on the road as the bus cd driver by? because that you, the internal bus light rays are moving to the front and ago of the bus in ~ the same constant speed of irradiate (due come the Physics postulate). Yet the distinction for you is the the ago of the bus is moving towards the light at the speed of the bus (let’s contact the bus’s speed ‘v’) and also the front of the bus is moving away indigenous the light at the rate of the bus. For this reason you view the light reach the ago before it reaches the front. If ‘Lₒᵤ’ is the street you see from the pear to each finish of the bus (which you’d mean to be the very same as Lₘₑ, however who knows in every this weirdness), the irradiate reaches the rear finish at time Lₒᵤ / (c + v) and also then the front end at time Lₒᵤ / (c -v). What I view as keep going simultaneously, you watch as keep going at various times!

Time slowly Down

I, top top the relocating bus, view my watch’s second hand ticking when every second. I pat the home window next come me top top every tick. You, on the street, watch my bus drive by with me top top it, tapping the home window periodically. You usage your watch to measure the time interval between my taps. You measure up one second between taps, right? Wrong! The time in between two taps as checked out by girlfriend is higher than one second! The elapsed time relies on the timekeeper.* friend will watch a moving clock as running slower than a stationary clock.*

What shenanigans deserve to I carry out on the bus this time to assist you know this? I notice that there’s a winter on the bus’s ceiling. I happen to have a flashlight. I suggest it up in the direction of the ceiling, switch it on and measure the moment it takes because that the light to reflect off the mirror and come earlier to me (the light from the winter comes back to me as soon as I check out the irradiate in the mirror). This just takes a few nanoseconds, so nearly speaking it’s no feasible because that me to measure up such a quick time interval. Yet imagine a tall sufficient bus and you acquire the idea. If ‘H’ is the elevation of the bus then ns measure a time of tₘₑ = 2H / c.

What time interval execute you measure for my flashlight beam to travel up to the ceiling, reflect off the mirror and return come me? girlfriend will see the irradiate beam go diagonally as much as the ceiling v the constant speed of irradiate (again due to the Physics postulate). And then reflect off and also return diagonally to me, again through the consistent light speed. Due to the fact that the diagonal courses are longer than the direct vertical courses (remember Pythagoras), you will measure the time interval in between me switching on the flashlight and also the irradiate beam reflecting earlier to me come be longer than what ns measured. You will see the time interval together dilated contrasted to what ns see.

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You observe the time passing on mine bus as slowed down by a factor of γ, which goes indigenous 1 (when the bus is stationary) to ∞ (when the bus move at the speed of light). Keep in mind that it’s no actually feasible for the bus to move at the speed of light because increasing to that rate would literally require limitless energy. Time dilation and other such results of one-of-a-kind Relativity become noticeable when γ is noticeably bigger than 1. It takes a yes, really high speed for γ to gain large. Because that example, at 87% the the speed of light, γ just around becomes larger than 2. In our daily lives, us don’t normally encounter such rapid objects.

Is this time dilation true or is it just some beguiling trick? that is fully and measurably true. For example, there space unstable, sub-atomic elementary particles dubbed *muons* that degeneration to half their quantity every 2 microseconds. These particles are current in cosmic beam bombarding the planet all the time. However, v a 2 microsecond degeneration period, we would suppose hardly any muons to endure the long journey through the earth atmosphere prior to reaching us on the earth’s surface. Yet in fact, many more than naively expected execute survive long enough. The reason for that is time dilation. Indigenous our view on earth, the muons room moving really fast, so their decay time dilates, top to considerably longer survive times.

One last note. There are countless so-called paradoxes connected with time dilation, casting doubt ~ above its validity (e.g. The twin paradox). Allow me reiterate together strongly as possible: time dilation is one observable and measurably true effect and does not actually lead to any kind of paradoxes whatsoever. Every alleged paradox is in reality a an outcome of an incomplete or incorrect knowledge of distinct Relativity.

Lengths Shorten

I measure up the length of the moving bus i’m on. From the street, you measure the size of my bus together it overcome you by. We both agree ~ above the bus’s length, right? Wrong! Lengths depend on the measurer.* You will observe the bus as much shorter than what i observe.* The size will it is in shortened by the factor γ us introduced in the time dilation ar above.

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This time mine setup on the bus will usage the same light and mirror as that used to highlight time dilation, except the mirror will certainly be placed at the front of the bus and also the light at the back of the bus. If ns measure the length of the bus to it is in ‘Lₘₑ’, then the moment it takes because that the light to reach the prior of the bus and also return come the ago of the bus will certainly be tₘₑ = 2 Lₘₑ / c.